fixes two small diffusion bugs

Summary: make sure we only print the fancy tool tip thing if we have all the data we need. (fixes T2136). additionally, default $branches to array() to prevent errors from reset on null.

Test Plan: made a checkin for a mercurial repo with user "foo@bar" with no branch. verified name showed up in all views. also noted on commit detail view there was no more error about reset() on null for $branches.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T2136

Differential Revision: https://secure.phabricator.com/D4065